Process for producing separator and separator

ABSTRACT

In fuel cell separator, the periphery of manifold through which fuel gas, reaction water, etc., pass and the seal line being a site of bonding with an adjacent separator are provided with a resin layer. Within the surface of the separator, the resin application site on which the resin layer is formed undergoes subbing treatment in advance to thereby increase the capability of bonding with the resin. When the resin layer consists of a resin having an NH group, as the subbing treatment, hydroxide deposition treatment is carried out on the surface of the separator.

This is a 371 national phase application of PCT/JP2007/053849 filed 22Feb. 2007, claiming priority to Japanese Patent Application No.2006-064744 filed 9 Mar. 2006, the contents of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to a separator that forms a partition wallbetween unit cells of a fuel cell, and a process for producing theseparator.

BACKGROUND OF THE INVENTION

Fuel cells are now being used in vehicles and in other fields. In asolid polymer fuel cell (hereafter simply referred to as a “fuel cell”),an assembly (MEA: Membrane Electrode Assembly) comprising an electrolytefilm formed from a solid polymer film sandwiched between two electrodes,namely a fuel electrode and an air electrode, is itself sandwichedbetween two separators to generate a cell that functions as the smallestunit, and a plurality of these cells are then stacked to form a fuelcell stack (FC stack), enabling a high voltage to be obtained.

Here, the fuel cell separators are formed from a metal material such asSUS. A multitude of techniques have been proposed for improving thecorrosion resistance of the separators formed from this metal. Forexample, JP 2002-25574 A and JP 2005-158441 A disclose a technique inwhich the peripheral edges of the separator manifold are covered with aresin or film having excellent corrosion resistance. The manifolddescribes an aperture formed in the separator, and the fuel gas, air andmoisture and the like pass through this manifold.

However, the surface of the separator is normally coated with achemically inert passivation film, and the bonding strength to resins orthe like is weak. As a result, the stress generated during cellstacking, and the stress generated due to the expansion that occursunder the action of the heat generated during operation of the fuel cellmay cause the resin layer or the like to peel. Furthermore, because thesurface of the separator has inherently poor wetting properties, if theresin coating material is water-soluble, then applying the resin coatingmaterial uniformly to the separator surface is impossible, and afavorable resin layer can not be formed.

JP 2000-243408 A discloses that when those regions of the fuel gaspassages formed within the center of the separator that do not contactthe electrode are coated with a resin, the bonding properties with theseparator can be improved by including OH groups within the resin.Furthermore, JP 2003-272655 A discloses a technique in which apassivation treatment is conducted by immersing the separator in anacidic bath in order to improve the performance of the fuel gaspassages. However, both of these techniques aim to improve the qualityand function of the fuel gas passages, and applying these techniques toimprovement of the quality and function of those regions of theseparator besides the gas passages is problematic.

In other words, with the conventional technology, effectively improvingthe corrosion resistance and the like of those regions of the separatorbesides the gas passages is problematic, and as a result, improving thequality of the fuel cell itself is difficult.

Accordingly, it is an advantage of the present invention to provide aseparator that is capable of improving the quality of a fuel cell, and aprocess for producing the separator.

SUMMARY OF INVENTION

A process for producing a separator according to the present inventionis a process for producing a separator that forms a partition wallbetween unit cells of a fuel cell, wherein on the separator substrate, asubbing treatment is performed within at least a resin applicationregion in which a resin layer is to be provided, to thereby allow ahydroxide or oxide having a high affinity for atoms constituting theresin to be deposited, prior to provision of the resin layer.

In a preferred aspect, the resin application region is in the vicinityof an opening through which a fluid can pass. In another preferredaspect, the resin application region is a seal line that represents aregion of bonding with an adjacent separator.

In those cases where the resin layer is composed of an NH-containingresin, the subbing treatment is preferably a treatment in which ahydroxide is deposited on the surface of the separator substrate. Inthis case, the subbing treatment is preferably a treatment in which OHgroups are covalently bonded to metal atoms on the surface of theseparator substrate. In those cases where the resin layer is composed ofan OH-containing resin, the subbing treatment is preferably a treatmentin which oxygen atoms are covalently bonded to metal atoms on thesurface of the separator substrate.

The subbing treatment is preferably performed only within the resinapplication region. The subbing treatment is preferably conducted suchthat on the separator substrate, those regions besides the resinapplication region are masked so as to inhibit the subbing treatment. Inanother aspect, the subbing treatment performed in those regions besidesthe resin application region is preferably removed following completionof the subbing treatment.

The resin layer is preferably formed by electrodeposition coating.Furthermore, the resin is preferably a water-soluble resin.

A separator that represents another aspect of the present invention is aseparator that forms a partition wall between unit cells of a fuel cell,wherein a resin layer is provided on those portions of the separatorsubstrate excluding the gas passages, for protecting the separatorsubstrate, and OH groups or O atoms that are chemically bonded directlyto the atoms on the surface of the separator substrate are alsochemically bonded directly to atoms that constitute the resin, betweenthe separator substrate and the resin layer.

According to the present invention, a resin application region on whicha resin layer is to be formed is subjected to a subbing treatment thatimproves the bonding strength between the resin and the separatorsubstrate, and therefore peeling of the resin layer is unlikely, and thecorrosion resistance of the separator can be maintained over a longperiod. As a result, the quality of the fuel cell can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing the structure of a fuel cell.

FIG. 2 is a top view of a separator.

FIG. 3 is a schematic illustration showing the state of bonding betweena separator and a resin prior to subbing treatment.

FIG. 4 is a schematic illustration showing the state of bonding betweena separator and a resin following completion of a subbing treatment.

FIG. 5 is a schematic illustration showing another state of bondingbetween a separator and a resin following completion of a subbingtreatment.

DETAILED DESCRIPTION

Embodiments of the present invention are described below with referenceto the drawings. First is a description of the structure of a solidpolymer fuel cell, with reference to FIG. 1. In a solid polymer fuelcell 10, an assembly (MEA: Membrane Electrode Assembly) comprising anelectrolyte film 12 formed from a solid polymer film sandwiched betweentwo electrodes, namely a fuel electrode 14 and an air electrode 16, isitself sandwiched between two separators 20 to generate a cell thatfunctions as the smallest unit, and a plurality of these cells are thennormally stacked together to form a fuel cell stack (FC stack), enablinga high voltage to be obtained.

The mechanism for power generation by the solid polymer fuel cell 10generally involves the supply of a fuel gas such as ahydrogen-containing gas to the fuel electrode (the anode side electrode)14, and supply of an oxidant gas such as a gas comprising mainly oxygen(O₂) or air to the air electrode (the cathode side electrode) 16. Thehydrogen-containing gas is supplied to the fuel electrode 14 via finechannels formed in the surface of the separator 20, and the action ofthe electrode catalyst causes the hydrogen to dissociate into electronsand hydrogen ions (H⁺). The electrons flow through an external circuitfrom the fuel electrode 14 to the air electrode 16, thereby generatingan electrical current. Meanwhile, the hydrogen ions (H⁺) pass throughthe electrolyte film 12 to the air electrode 16, and bond with oxygenand the electrons that have passed through the external circuit, therebygenerating reaction water (H₂O). The heat that is generated at the sametime as the bonding reaction between hydrogen (H₂), oxygen (O₂) and theelectrons is recovered using cooling water. Furthermore, the watergenerated at the air electrode 16 on the cathode side of the assembly(hereafter referred to as “reaction water”) is discharged from thecathode side.

FIG. 2 is a top view of the separator 20. The two separators 20 thatsandwich the MEA 15 perform a role as partitions for separating thehydrogen gas and the oxygen gas, and also have a function ofelectrically connecting the stacked cells in a series arrangement. Theseseparators 20 are formed from a metal material such as SUS, and can beformed by press molding or the like.

Gas passages 22 that allow the passage of the hydrogen-containing gasand the oxygen-containing gas or air are formed in the center of eachseparator 20. Fine uneven channels are formed within the gas passages 22shown by the hatching in FIG. 2, and the fuel gas is supplied to theassembly via these channels.

Openings that allow the passage of the fuel gas, cooling media, andreaction water and the like are formed as a manifold 24 near the leftand right edges of the separator 20. The region in the periphery of thismanifold 24 is affected by the fuel gas and water and the like passingthrough the manifold 24, and is therefore a corrosion-prone environment.Corrosion around the periphery of the manifold 24 not only reduces thelife of the fuel cell, but also causes a reduction in the cellefficiency. In other words, corroded substances detaching and adheringto the inner wall surfaces of the manifold 24 can cause partial blockingof the manifold 24, resulting in a deterioration in the flow of the fuelgas and the like. As a result, the formation of a corrosion-resistantresin layer around the periphery of the manifold 24 has already beenproposed.

Furthermore, the outer periphery of the separator 20 functions as thesite of bonding with an adjacent separator 20, a so-called seal line 26.Each separator 20 is bonded to the adjacent separator 20 via an adhesivethat is applied to this seal line 26. However, the bonding strengthbetween the separator 20 and the adhesive is usually weak. Accordingly,coating the seal line 26 with a member such as a resin that exhibitspowerful bonding to the adhesive, thereby improving the bonding strengthbetween separators 20, has already been proposed.

In this manner, the surface of the separator 20 is often coated with aresin in order to improve the corrosion resistance or improve theadhesion with the adjacent separator 20. However, as shown in FIG. 3, achemically inert passivation film 20 a is often formed on the surface ofthe separator 20. This passivation film 20 a is known to exhibit pooraffinity with a resin 50. As a result, even if resin coating isperformed, if a passivation film 20 a exists then the bonding strengthbetween the resin 50 and the separator 20 is weak, and peeling of theresin 50 tends to occur relatively readily under the stress generatedduring cell stacking, or the stress that is generated accompanying thethermal expansion that occurs during operation of the fuel cell. Peelingof the resin 50 then induces corrosion of the separator 20. Furthermore,resin 50 that has peeled and detached causes a deterioration in theefficiency of the fuel cell. These factors tend to invite adeterioration in the quality of the fuel cell.

Furthermore, in recent years, water-soluble resins have become widelyused as a result of their ease of handling, but because the surface of aseparator 20 covered with an inert passivation film 20 a has poorwettability, it tends to repel water-soluble resins. As a result,uniform spreading of the resin coating material is inhibited, and thethickness of the resin layer may lack uniformity. Furthermore, very fineholes (pinholes) tend to occur in the formed resin layer, meaning aportion of the separator surface that is supposed to be covered by theresin layer may be exposed externally.

In an embodiment of the present invention, in order to prevent theseproblems and enable a higher quality separator to be obtained, prior tothe formation of the resin layer, a subbing treatment is performedwithin the resin application region where the resin layer is to beformed, thereby depositing a hydroxide or oxide with a high affinity forthe resin on the surface of the separator.

In the present embodiment, because a resin layer is formed over theentire surface of the separator 20 excluding the gas passages 22, thisregion is specified as the resin application region. However, a limitedportion may also be specified as the resin application region, with thesubbing treatment and resin layer formation then performed only withinthis region. In such a case, at least the periphery around the manifold24 that requires a high level of corrosion resistance, or the seal line26 along which resin layer formation is preferably conducted in order toimprove the adhesion is preferably specified as the resin applicationregion. Furthermore, because a resin layer cannot be formed within theregion of the gas passages 22 through which the fuel gas and the likeflow, this region should not be specified as the resin applicationregion.

FIG. 4 is a schematic illustration showing the state of bonding betweena separator 20 that has undergone a subbing treatment and an NHgroup-containing resin 50. An NH group-containing resin 50 that containsNH groups generally exhibits a high degree of affinity with OH groups.If a subbing treatment is used to deposit a hydroxide onto the surfaceof the separator 20 in advance, then the OH groups that exist at thesurface of the separator 20 and the NH groups contained within the resin50 undergo a mutual attraction, and bond powerfully together. As aresult, even upon exposure to the stress generated during stacking, theresin layer will not peel, and the resin-coated state can be maintained.

This effect also applies in the case where the resin layer 50 is formedusing an OH group-containing resin on the surface of a separator 20 ontowhich an oxide has been deposited via a subbing treatment. FIG. 5 is aschematic illustration showing the state of bonding between a separator20 that has undergone a subbing treatment and an OH group-containingresin 50. An OH group-containing resin 50 that contains OH groupsgenerally exhibits a high degree of affinity with oxygen atoms.Accordingly, if a subbing treatment is used to deposit an oxide onto thesurface of the separator 20 in advance, then the adhesion between theresin 50 and the separator 20 can be improved, meaning peeling of theresin can be prevented even upon exposure to the stress generated duringstacking.

Furthermore, in the present embodiment, the OH groups (or O atoms) thatare chemically bonded to the atoms on the surface of the separator 20bond chemically to the atoms that constitute the resin 50. In otherwords, the separator 20 and the resin 50 can be claimed to be bonded atthe atomic level. Accordingly, unlike the conventional technology wherea corrosion-resistant member is affixed to the separator surface via anadhesive or the like, in the present embodiment, the gap between theseparator 20 and the resin 50 can be reduced to effectively nothing. Asa result, penetration of water or the like into the space between theresin 50 and the separator 20 can be reliably prevented, enabling thecorrosion resistance of the entire separator 20 to be effectivelyimproved.

Moreover, when a hydroxide or an oxide is deposited on the surface ofthe separator 20, the wettability (hydrophilicity) of the surface canalso be improved. As a result, even when a water-soluble resin isapplied to the surface of the separator 20, the resin is not repelled,and a favorable resin layer can be obtained that has a uniform thicknessand suffers no problems such as pinholes or the like.

Next is a more detailed description of the subbing treatment performedon the surface of the separator 20. There are no particular restrictionson the subbing treatment performed on the separator 20, provided thetreatment is capable of depositing a hydroxide or oxide onto theseparator surface, and one suitable example is a plasma treatment. Asalready known, a plasma treatment involves bringing a plasma-state gasinto contact with the member undergoing treatment, thereby inducing achemical reaction between the gas and the member. According to such aplasma treatment, the region altered by the chemical reaction can belimited to only a thin layer having a submicron thickness from thesurface of the member undergoing treatment. In order to perform asubbing treatment on the surface of the separator 20 using this type ofplasma treatment, the separator 20 is placed within an atmospherecontaining a plasma-state gas. In such a case, a chemical reaction isinduced between water (H₂O) within the atmosphere and the surface layerof the separator 20. As a result, chemically active OH groups aredeposited on the surface of the separator 20 instead of the inertpassivation film.

Furthermore, chemical conversion treatment is another example of apossible subbing treatment method. In this method, a chemical reactionis initiated electrically with the separator 20 immersed within asolvent containing O atoms, thereby depositing an oxide on the separatorsurface. In this case, covalent bonds are formed between the metal atomsat the surface of the separator 20 and the oxygen atoms, and those bondsare very strong. As a result, the bonding strength between the separator20 and the resin 50 also improves, and peeling of the resin 50 can beprevented even more reliably.

When a subbing treatment such as those described above is performed, thegas passages 22 are first masked, so as to inhibit subbing treatment ofthe gas passages 22. This masking is to prevent any deterioration in thecorrosion resistance that may accompany the subbing treatment. Namely,when a subbing treatment is performed, the passivation film is removed.The periphery around the manifold 24 and the seal line 26 where theresin layer is formed are protected by the resin layer, and thereforeremoval of the passivation film presents no particular problems. Howeverin the region of the gas passages 22, which is not protected by theresin layer, removal of the passivation film causes a dramatic reductionin the corrosion resistance. Such corrosion invites an increase in thesurface resistance and detachment of corroded substances, leading to adeterioration in the efficiency of the fuel cell. Accordingly, in thepresent embodiment, the gas passages 22 are masked prior to performingthe subbing treatment. This masking can be achieved by a technique inwhich a sealing film that inhibits penetration of the plasma or thesolvent is bonded in a removable manner on top of the gas passages 22.

The corrosion resistance of the gas passages may also be maintained notby preventing subbing treatment of the gas passages 22, but rather byremoving the subbing treatment performed on the gas passages 22. Inother words, the subbing treatment may be performed upon the entireseparator 20, without any masking, with a passivation film then beingre-formed on the surface of the gas passages 22.

Provided the subbing treatment enables a hydroxide or oxide to bedeposited on the surface of the resin application region, a resin layercan then be formed favorably within the resin application region. Theresin layer can be formed using any of a variety of conventionaltechniques, although the following description focuses on resin layerformation by electrodeposition coating. In those cases where the resinlayer is formed by electrodeposition, the masked separator that hasalready undergone subbing treatment is immersed as the cathode within aresin coating material. A direct electrical current is then passedbetween the separator (the cathode) and an anode (+) positioned inside amembrane chamber within the electrodeposition tank, thereby forming aresin layer on the surface of the separator. At the time of thiselectrodeposition coating, the surface of the separator is in a statewherein a hydroxide or oxide that exhibits favorable affinity with theresin coating material has already been deposited on the surface (seeFIG. 4 and FIG. 5). Accordingly, the resin coating material bondsstrongly to the OH groups (or O atoms) on the separator surface. As aresult, peeling of the resin can be more reliably prevented, and thecorrosion resistance of the separator can be maintained over a longperiod. Furthermore, because the resin and the separator are bondedtogether at the atomic level, the gap between the two can be reduced toeffectively nothing, enabling a high degree of corrosion resistance tobe achieved. Moreover, because the hydrophilicity of the separator isimproved as a result of the subbing treatment, a resin layer of uniformthickness can be obtained without the occurrence of problems such aspinholes.

A separator production apparatus used for producing the separator isequipped with a subbing treatment device, namely an aforementionedplasma treatment device or a chemical conversion treatment device or thelike, that is capable of performing a subbing treatment on the resinapplication region of a separator substrate that has been molded to apredetermined shape. The resin layer may either be formed immediatelyfollowing the subbing treatment, or formed immediately prior tosandwiching the MEA or the like between the separators.

As described above, according to an embodiment of the present invention,a high-quality separator can be obtained that is capable of maintaininga high degree of corrosion resistance over a long period.

The invention claimed is:
 1. A process for producing a separator thatforms a partition wall between unit cells of a fuel cell, the processconsisting of: performing a subbing treatment only within a resinapplication region on a separator substrate, the resin applicationregion being a region in which a resin layer is to be provided, todeposit a hydroxide or oxide having an affinity for atoms constitutingthe resin to be deposited, prior to provision of the resin layer; andforming the resin layer within the resin application region that hasbeen subjected to the subbing treatment, wherein the separator substratehas gas passages formed thereon, and wherein the resin applicationregion excludes the gas passages.
 2. The process for producing aseparator according to claim 1, wherein the resin application region isin a vicinity of an opening through which a fluid can pass.
 3. Theprocess for producing a separator according to claim 2, wherein theresin application region is a seal line that represents a region ofbonding with an adjacent separator.
 4. The process for producing aseparator according to claim 3, wherein in a case where the resin layercomprises an NH group-containing resin, the subbing treatment is atreatment in which a hydroxide is deposited on a surface of theseparator substrate.
 5. The process for producing a separator accordingto claim 4, wherein the subbing treatment is a treatment in which OHgroups are covalently bonded to metal atoms on the surface of theseparator substrate.
 6. The process for producing a separator accordingto claim 3, wherein in a case where the resin layer comprises an OHgroup-containing resin, the subbing treatment is a treatment in whichoxygen atoms are covalently bonded to metal atoms on the surface of theseparator substrate.
 7. The process for producing a separator accordingto claim 3, wherein following the subbing treatment, the subbingtreatment performed in regions besides the resin application region isremoved.
 8. The process for producing a separator according to claim 3,wherein the resin layer is formed by electrodeposition coating.
 9. Theprocess for producing a separator according to claim 3, wherein theresin is a water-soluble resin.
 10. The process for producing aseparator according to claim 1, wherein the subbing treatment isperformed such that, on the separator substrate, regions besides theresin application region are masked so as to inhibit the subbingtreatment.